A plurality of white light-emitting diodes are used for a backlight of a liquid crystal display apparatus included in a mobile electronic apparatus such as a cellular phone. A conventional method using a constant-current drive is generally used to cause the plurality of white light-emitting diodes to emit light with even luminance.
FIG. 1 illustrates a light-emitting diode (hereinafter referred to as an LED) drive circuit using the conventional constant-current drive method. As shown in FIG. 1, the LED drive circuit includes an LED LED101 and a resistor R101 having a resistance value r101. When iL (not shown) and Vc represent a drive current and a reference voltage of the LED LED101, respectively, iL is equal to Vc divided by r101 (i.e., iL=Vc/r101).
In the LED drive circuit, the drive current iL of the LED LED101 is controlled so that a voltage drop by the resistor R101 becomes equal to the reference voltage Vc. Therefore, a battery voltage Vbat needs to be larger than a forward voltage VF of the LED LED101 added to the reference voltage Vc. Further, by taking into account the fact that the battery voltage Vbat decreases in the course of use, the battery voltage Vbat needs to be much larger than the forward voltage VF of the LED LED101 added to the reference voltage Vc. As a result, the amount of electricity consumed by components other than the LED LED101 increases, thereby impairing efficiency in power supply.
FIG. 2 illustrates another conventional LED drive circuit. As shown in FIG. 2, the LED drive circuit includes an LED LED111, a resistor R11, a charge pump circuit 111, an LED inactive state detection circuit 112, and a switching control circuit 113.
The charge pump circuit 111 is used as a power source for the LED LED111 so as to attempt to eliminate an influence of fluctuations in the battery voltage Vbat on the LED LED111. The switching control circuit 113 controls switching of the LED LED111 between an active state and an inactive state. The LED inactive state detection circuit 112 detects a state of the LED LED111. When the inactive state of the LED LED111 is detected, an enable signal is turned off to stop operation of the charge pump circuit 111 to attempt to improve efficiency in power supply.